1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic tape and a magnetic disk. More particularly, it relates to a magnetic recording medium which uses a polyurethane resin having excellent properties, as a resinous binder used in a magnetic layer formed on a non-magnetic support.
2. Description of the Prior Art
In general, magnetic recording media such as magnetic tapes are produced by applying a magnetic coating composition to a polyethylene terephthalate film. The magnetic coating composition is usually prepared by dispersing needle-like magnetic particles having a long axis with a length of 1 .mu.m or less in a solution of a resinous binder together with appropriate additives such as dispersing agents, lubricating agents, and antistatic agents.
The resinous binders used in the magnetic recording media are required, not only to yield excellent dispersibility, filling efficiency, and orientation of magnetic particles, but also to provide the magnetic layer with excellent durability, abrasion resistance, heat resistance, and adhesion to the non-magnetic support. Thus, the resinous binders play a very important role. Examples of conventional resinous binders which have been hitherto used include vinyl chloride/vinyl acetate copolymers, vinyl chloride/vinyl acetate/vinyl alcohol copolymers, vinyl chloride/vinylidene chloride copolymers, polyurethane resins, polyester resins, acrylonitrile/butadiene copolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins, and acrylic resins.
In the magnetic recording media, in order to improve the S/N ratio (signal/noise ratio) and realize a high recording density, a high orientation and a smooth surface of the magnetic layer by filling more finely ground magnetic particles in the magnetic layer, or an adoption of magnetic particles which are hardly dispersible such as metal particles and barium ferrite particles have been tried in order to achieve these ends. Because of this, resinous binders which have satisfactory dispersibility are required. Moreover, in order to make the surface of the magnetic layer smooth, satisfactory running durability is required for resinous binders.
In addition, the above-mentioned demand for magnetic recording media is increasing along with the recent improvement of magnetic recording technique. Therefore, in actuality, the conventional polyurethane resin cannot satisfy the demand.